The present invention relates to metal plated high heat ABS articles prepared from injection molded substrates which contain substantial amounts of alphamethyl styrene and which also contain a small amount of an alkyl substituted silicone polymer. The plated articles of the present invention have both excellent heat deflection properties and thermal cycle properties.
"High Heat ABS" materials have been available for molding applications since the early 1960's. Such materials are usually blends of alphamethyl styrene-acrylonitrile polymers and ABS graft polymers and are disclosed, for example, in U.S. Pat. Nos. 3,010,936 and 3,111,501. The alphamethyl styrene-acrylonitrile polymer imparts improved dimensional stability at elevated temperatures because of its higher glass transition temperature compared to the styrene-acrylonitrile copolymer of "Standard ABS" materials. In order for the practical use temperature range, or the ASTM heat deflection temperature, to be significantly higher than "Standard ABS," the level of incorporation of alphamethyl styrene must be very high. For example, to attain a 212.degree. F. heat deflection temperature at 264 psi using a 1/2.times.1/2 inch, unannealed, compression molded specimen in the ASTM D-648 test, about 35% of the total polymer or 40% of the copolymer (non-rubber) portion must consist of alphamethyl styrene. Commercial materials having 221.degree. F. ASTM heat deflection values may have 45% or more alphamethyl styrene content. The "high heat ABS" materials would be expected to deviate from simple analogies to standard ABS made with styrene-acrylonitrile copolymer. Alphamethyl styrene exhibits very different polymer chemistry than styrene because of its disubstituted olefinic nature. This affects both effective synthesis techniques and polymer properties. For example, styrene-acrylonitrile copolymer deviates positively from the Fox equation glass transition temperature vs. composition prediction, whereas alphamethyl styrene-acrylonitrile copolymer shows a large negative deviation.
It is not surprising, therefore, that no successful plating grade high heat ABS, having a heat deflection temperature above 212.degree. F. and capable of passing automotive industry thermal cycle tests, has been invented, even though standard ABS grades have been available for several years which can meet such tests. In fact, early high heat ABS plating grades with a 220.degree. heat deflection temperature have been suggested (U.S. Pat. No. 3,445,350) and tried, but were not successful in thermal cycle tests of injection molded parts. Alloys or blends of high heat ABS and polycarbonate have also been commercially available and are well known. See, for example, Australian Pat. No. 441,164. The metal plating of such alloys was also suggested in U.S. Pat. No. 3,445,350. However, such materials were not capable of passing automotive thermal cycle tests, in which failures occur at low temperatures.
The use of silicone additives to improve the impact strength of graft ABS is disclosed in U.S. Pat. No. 3,046,239. In this patent, alphamethyl styrene is disclosed as a comonomer in the graft ABS. However, there is no disclosure of metal plating the ABS, nor the effect of such additives on the thermal cycle performance of metal plated high heat ABS. Further, the use of organosilicones in standard ABS resins to permit lower amounts of organo metallic soaps used as lubricants in the resin system is taught by Japanese publication 49-29947.
A high heat ABS material, or family of materials, including ABS/Polycarbonate alloys, would extend the useful range of plated part performance to higher temperatures. This would be of use in the plating operation where some parts distort in the 160.degree. F. plating etch bath, causing misfits in assembly. The plated part would also be useful at higher temperatures, such as encountered in touch-up ovens. This would eliminate or reduce the need for insulating devices during touch-up painting.